我什么时候可以将函数绑定到另一个名字? [英] When can I bind a function to another name?
问题描述
在解释器中工作时,将函数绑定到名称通常很方便,例如:
ghci>让f =(+1)
ghci> f 1
2
这会将名称 f
到函数(+ 1)
。简单。
然而,这并不总是奏效。我找到一个导致错误的例子,试图从 Data.List
模块中别名 nub
。例如,
ghci> :m Data.List
ghci> nub [1,2,2,3,3,3]
[1,2,3]
ghci>让f = nub
ghci> f [1,2,2,3,3,3]
< interactive>:1:14:
(Num())
文字'3'
可能的修正:为(Num())
添加一个实例声明在表达式中:3
在'f'的第一个参数中,即`[1,2, 2,3,...]'
在表达式中:f [1,2,3,...]
然而,如果我明确地声明 x
这个参数,那么它就不会有错误:
ghci>让f x = nub x
ghci> f [1,2,2,3,3,3]
[1,2,3]
任何人都可以解释这种行为吗?
在当前Ghci版本中输入默认规则有点难以理解。
您可以为 f
提供类型签名。或者按照Chris的建议,将:set -XNoMonomorphismRestriction
添加到〜/ .ghci
文件中。
When working in the interpreter, it's often convenient to bind a function to a name, for example:
ghci> let f = (+1)
ghci> f 1
2
This aliases the name f
to the function (+1)
. Simple.
However, this doesn't always work. One example I've found which causes an error is trying to alias nub
from the Data.List
module. For example,
ghci> :m Data.List
ghci> nub [1,2,2,3,3,3]
[1,2,3]
ghci> let f = nub
ghci> f [1,2,2,3,3,3]
<interactive>:1:14:
No instance for (Num ())
arising from the literal `3'
Possible fix: add an instance declaration for (Num ())
In the expression: 3
In the first argument of `f', namely `[1, 2, 2, 3, ....]'
In the expression: f [1, 2, 2, 3, ....]
However, if I explicitly state the argument x
then it works without error:
ghci> let f x = nub x
ghci> f [1,2,2,3,3,3]
[1,2,3]
Can anyone explain this behaviour?
Type defaulting rules in current Ghci versions are somewhat inscrutable.
You can supply a type signature for f
. Or add :set -XNoMonomorphismRestriction
to your ~/.ghci
file as was advised by Chris earlier.
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